In recent years high quality colour printers have become the norm. For ink jet printers, typical resolutions are 1200 dpi or higher, which translates into a printer ink dot size (and separation) of 20 microns or less. In many systems the ink jet printer may overprint regions multiple times to help minimise the effect of printer defects such as blocked printer head nozzles. The optical density of a printed colour can be very sensitive to the precise value of the displacement between overprinted regions. This means that (for high quality at least) it is necessary to control or calibrate the exact displacement of the printer head between overprints.
Many approaches have been proposed for calibrating the movements of the print head relative to the medium being printed upon. One approach to the calibration of print head position is the measurement of individual dot positions. Unfortunately, despite the simple experimental set-up and straightforward result analysis of this approach, it is quite unreliable due to the large variations in dot shape, position and size. There is also the difficulty of unambiguously locating isolated dots in large regions on the medium being printed upon.
More robust methods have also been suggested to accommodate the noise and ambiguity in order to achieve accurate measurement of print head position. Some methods measure the position of a print head by printing specially designed test charts and scanning the printed image later to find the relative shift of each overprint using Fourier analysis. Although these methods are robust to noise, they involve complex computation and are not performed in real-time due to the separate printing and scanning processes that are involved.
Other methods include a positioning method described in U.S. Pat. No. 6,568,787 where an optical sensor is used to accurately position the print head in the capping area of the service station. However, to determine the position of the sensor relative to the print head, a separate measurement has to take place beforehand through printing and scanning a specially designed test chart.
Other methods include a method described in US 2009/0268254 where an optical sensor is used to measure and correct print density error. However, the printing area to be corrected needs to be coved by the field of view of the sensor. Therefore, this method can be costly and require a considerable amount of computational power. Moreover, some print defects caused by linefeed error cannot be corrected in this way.